Update CONTRIBUTING.md guidelines

Signed-off-by: Carlos Rodriguez Hernandez <carlosrh@vmware.com>

bitnami/gitlab-runner-helper/CONTRIBUTING.md

@@ -17,6 +17,31 @@ When submitting a PR make sure that:
 - The title of the PR is clear enough.
 - If necessary, add information to the repository's `README.md`.
 
+#### Sign Your Work
+
+The sign-off is a simple line at the end of the explanation for a commit. All commits needs to be signed. Your signature certifies that you wrote the patch or otherwise have the right to contribute the material. The rules are pretty simple, you only need to certify the guidelines from [developercertificate.org](https://developercertificate.org/).
+
+Then you just add a line to every git commit message:
+
+    Signed-off-by: Joe Smith <joe.smith@example.com>
+
+Use your real name (sorry, no pseudonyms or anonymous contributions.)
+
+If you set your `user.name` and `user.email` git configs, you can sign your commit automatically with `git commit -s`.
+
+Note: If your git config information is set properly then viewing the `git log` information for your commit will look something like this:
+
+```
+Author: Joe Smith <joe.smith@example.com>
+Date:   Thu Feb 2 11:41:15 2018 -0800
+
+    Update README
+
+    Signed-off-by: Joe Smith <joe.smith@example.com>
+```
+
+Notice the `Author` and `Signed-off-by` lines match. If they don't your PR will be rejected by the automated DCO check.
+
 ### PR Approval and Release Process
 
 1. Changes are manually reviewed by Bitnami team members usually within a business day.